The invention relates to siliciding thermostructural composite materials.
Thermostructural composite materials are known for their good mechanical properties and their ability to conserve those properties at high temperature. Such materials are typically carbon/carbon (C/C) composites having carbon fiber reinforcement and a carbon matrix, and ceramic matrix composites (CMCs) with fiber reinforcement made of refractory fibers (in particular carbon fibers or ceramic fibers) and a ceramic matrix, or a matrix both of carbon and of ceramic (e.g. a matrix of silicon carbide SiC or a combined C/SiC matrix).
Parts made of C/C or CMC material are made by preparing a fiber structure or “preform” of a shape close to that of the part to be made, and densifying the preform with the carbon or ceramic matrix. Densification can be performed by a liquid technique or by a gas technique. The liquid technique consists in impregnating the preform with a liquid composition containing a precursor for the matrix, typically a resin. The precursor is transformed by heat treatment, thereby pyrolyzing the resin. The gas technique consists in performing chemical vapor infiltration (CVI). The preform is placed in an oven into which a reaction gas is introduced. The pressure and temperature conditions in the oven are adjusted so as to enable the gas to diffuse within the fiber preform and form a deposit of matrix material on the fibers, either by one of the components of the gas decomposing, or else by a reaction taking place between a plurality of components. Those methods of densification by a liquid technique or a gas technique are well known in themselves, and they can be associated with each other.
Whatever the fabrication method used, thermostructural composite materials present residual pores constituted by pores of greater or smaller size (macropores and micropores) that communicate with one another.
Proposals have been made to finish off the densification of thermostructural composite materials by siliciding, i.e. by introducing molten silicon. The object is to modify the thermomechanical characteristics of the materials, e.g. by increasing thermal conductivity or by making the materials more leakproof and/or reducing the cost of final densification, since the conventional method employing the liquid technique or the gas technique then does not need to be continued for the time required to obtain the maximum density that is possible by the method.
Depending on the nature of the composite material, siliciding may be rectional or non-rectional. An example of rectional siliciding, as described in particular in U.S. Pat. No. 4,275,095, consists in taking a composite material having a matrix comprising carbon at least in an outer phase of the material, and in impregnating it with molten silicon that then reacts with the carbon in order to form silicon carbide. An example of non-rectional siliciding is using molten silicon to impregnate a composite material in which the matrix is made of silicon carbide, at least in an outer phase of the matrix, i.e. a composite material in which the outer geometrical surface and the surfaces of the pores communicating with the outside are made of silicon carbide.
Molten silicon is very fluid and possesses high wetting ability, particularly on surfaces of carbon or silicon carbide. When a thermostructural composite material is impregnated with silicon in the liquid state, the silicon advances into the array of pores in the material following the surfaces of the pores. As shown very diagrammatically in FIG. 1, micropores and narrow passages or constrictions in the material M are filled in, however macropores are not filled in since the silicon (Si) flows along their surfaces. The extent to which the pores are filled in is thus random, which means that it is not possible to control thermal diffusivity and leakproofing. In addition, occluded gas pockets are formed that constitute inaccessible closed pores such as P.
Methods have been proposed for filling the pores of the composite material in part before performing infiltration with molten silicon.
Thus, document EP 0 835 853 proposes impregnating the material with an organic resin and performing heat treatment to pyrolize the resin. Nevertheless, the grains of carbon (resin coke) that are obtained are to be found not only in the macropores where they occupy part of their volume, but also in micropores or in constrictions in the array of pores. Under such circumstances, while siliciding, the silicon reacts with the carbon of the grains, thereby increasing their volume and closing off a pore, thereby preventing the silicon from passing. This results in siliciding that is irregular. Furthermore, in particular in the macropores, there remains a carbon phase that is sensitive to oxidation and that is constituted by the resin coke grains that have not been silicided or that have not been silicided sufficiently.
Proposals are also made in document U.S. Pat. No. 5,865,922 to impregnate the thermostructural composite material with a resin having a relatively high coke content together with a pore-generating agent. This agent serves to form a foam prior to polymerization of the resin, so pyrolysis gives a carbon residue that is porous, and that is subsequently impregnated with silicon. That method likewise does not guarantee uniform filling of the initial pores in the composite material by siliciding. While the foam is forming, the resin can flow back out from the material leading to a variable resin content in the material, and irregular porosity in the porous residue that results from pyrolyzing the resin. Furthermore, the transformation into foam can itself be irregular, with relatively large grains of carbon residue being formed that are not silicided in full, and with closed pores being formed in the foam that remain inaccessible to the silicon.